The present invention relates to a method in a wireless communication system as set forth in the preamble of the appended claim 1. The invention relates also to a wireless communication system as set forth in the preamble of the appended claim 15.
A considerable increase in the use of information services as a result of an increase in particularly the Internet and so-called World Wide Web (WWW) services has brought about a need to develop faster communication services for transferring information between a provider of an information service and a terminal using the information service. Furthermore, several information services also comprise so-called multimedia information, such as images, video images, and sound. The transmission of such multimedia information requires a high data transmission rate to execute the data transmission as close to real time as possible.
Communication systems intended for an office environment, so-called local area networks (LAN), are to a great deal implemented as wired systems. Thus, the data transmission connection between terminals and a server is implemented either electrically via a cable or optically via an optical fibre. It is an advantage of such a fixed system that relatively high data transmission rates can be achieved. A drawback in such a fixed communication network is that it is difficult to make changes, and the terminals must usually be placed relatively close to connection points intended for them, whereby the movability of the terminal is affected. The implementation of such a wired local area network in an already existing building is not always successful, or the wiring of cables afterwards is expensive. On the other hand, a communication cable system which may already exist particularly in older buildings is not necessarily suitable for fast data transmission.
For implementing local area networks, there are several wireless communication systems under development. Several wired communication systems are based on the use of radio signals in data transmission.
One such communication system for a local area network based on radio communication is the so-called HIPERLAN (Hlgh PErformance Radio Local Area Network). Such a radio network is also called a broadband radio access network (BRAN).
In version 2 of the HIPERLAN communication system under development, the aim is to achieve a data transmission rate of even more than 30 Mbit/s, the maximum cell range being some tens of metres. Such a system is suitable for use in the same building e.g. as an internal local area network for one office. There is also a so-called HIPERACCESS communication system under development, in which an aim is to achieve the same data transmission rate as in said HIPERLAN/2 communication system but with the cell range of some hundreds of metres, wherein the HIPERACCESS system is applicable for use as a larger local area network e.g. in schools and larger building complexes.
In the HIPERLAN/2 system which is used as an example, the frame structure used in the data link layer DLC is shown in a reduced manner in the appended FIG. 1b. The data frame FR consists of control fields C, such as RACH (Random Access CHannel), BCCH (Broadcast Control CHannel) and FCCH (Frame Control CHannel), as well as a data field D which comprises a given number of time slots TS1 , TS2, . . . , TSsn, in which it is possible to transmit actual payload information.
Each control field C as well as the packets to be transmitted in the time slots of the data field preferably comprise error checking data which has been calculated by an access point AP1 that transmits the data frame and added into the control fields C of the data frame and to the packets to be transmitted in the time slots TS1, TS2, . . . , TSn. This checking data is preferably a checksum calculated on the basis of information contained in said field, such as CRC (Cyclic Redundancy Check). In the receiving wireless terminal MT1, it is possible to use the error checking data to examine if the data transmission possibly contained any errors. There can also be several such error check data in the field C, D, calculated on part of the information contained in the field. For example in the HIPERLAN/2 system, the FCCH control field consists of smaller information elements, for which error checking data is calculated respectively. The number of these information elements may vary in each data frame. All data frames do not necessarily have an FCCH control field, in which case the number of information elements is zero.
Communication in the HIPERLAN/2 system is based on time division multiple access TDMA, wherein there can be several connections simultaneously on the same channel, but in said frame each connection is allotted a time slot of its own, in which data is transmitted. Because the quantity of data to be transmitted is usually not constant in all the simultaneous connections, but it varies in time, a so-called adapted TDMA method is used, in which the number of time slots to be allocated for each data transmission connection may vary from zero to a maximum, depending on the loading situation at each time as well as on the data transmission capacity allocated for the connection.
For the time division multiple access to work, the terminals coupled to the same node must be synchronized with each other and with the transmission of the node. This can be achieved for example in such a way that the receiver of the wireless terminal receives signals on a channel. If no signal is detected on the channel, the receiver shifts to receive on another channel, until all the channels are examined or a channel is found on which a signal is detected that is transmitted from an access point. By receiving and demodulating this signal, it is possible to find out the time of transmission of the control channel BCCH of the access point in question and to use this to synchronize the terminal. In some cases, the terminal may detect a signal from more than one access points, wherein the terminal preferably selects the access point with the greatest signal strength in the receiver and performs synchronization to this access point.
After the terminal is synchronized to the access point, the terminal can start a connection set-up to couple to this access point. This can be performed preferably so that the terminal transmits a connection set-up request to the access point on the RACH control channel. In practice, this means that the terminal transmits in a time slot allocated for the RACH control channel and the access point simultaneously listens to communication on the channel, i.e. receives signals on the channel frequency used by the same. After detecting that a terminal is transmitting a connection set-up request message, the access point takes the measures required for setting up the connection, such as resource allocation for the connection, if possible. In the resource allocation, the quality of service requested for the connection is taken into account, affecting e.g. on the number of time slots to be allocated for the connection. The access point informs the terminal if the connection set-up is possible or not. If it has been possible to set up a connection, the access point transmits in the BCCH control field information e.g. on the transmission time slots, receiving time slots, connection identifier, etc. allocated for the connection. The number of transmission and receiving time slots is not necessarily the same, because in many cases the quantity of information to be transmitted is not the same in both directions. For example, when an Internet browser is used, considerably less information is transmitted from the terminal than information is received to the terminal. Thus, for the terminal, fewer transmission time slots are needed than receiving time slots. Furthermore, the number of time slots allocated for the connection may preferably vary in different frames according to the need to transmit information at the time. The access point controller is provided with a so-called scheduler, which serves e.g. the purpose of allocating time slots for different connections as mentioned above. The scheduler is implemented preferably in an application program in the access point controller.
Because full-duplex communication is needed in local area networks, also a full-duplex data transmission connection is needed on the radio channel. In a time division system this can be implemented either in such a way that some of the time slots in a frame are allocated for transmission from the wireless terminal to the access point (uplink) and some are allocated for transmission from the access point to the wireless terminal (downlink), or in such a way that a separate frequency band is allocated for each communication direction. In the HIPERLAN/2 system, the introduction of the first mentioned method is proposed, wherein the access point and the terminals coupled therewith do not transmit simultaneously.
In the HIPERLAN/2 systems, the access points can select the channel to be used in the connection irrespective of the other access points. Furthermore, the scheduler of the access point selects the moment of time to be used for the transmission irrespective of the other access points. In practice, this means that two or more access points can make an attempt to transmit simultaneously on the same channel, wherein the transmission is unsuccessful. In order to prevent this collision of transmissions, the transmitting access point or wireless terminal first listens to the communication of that channel on which the transmission is conducted. If no communication is detected on the channel within a given period of time, it is presumed that the channel is free and the transmission can be started. However, if communication is detected on the channel, the receiver is synchronized with this transmission. When the transmission is terminated, a possible new message is waited for and after that it is possible to retry to obtain the channel. However, several access points and/or wireless terminals may be waiting for their transmission turn, wherein situations may occur where several devices make an attempt of transmitting simultaneously. Furthermore, situations may arise in which all the devices on a particular channel are waiting for a transmission turn, i.e. the channel is under utilized. On the other hand, information is not transmitted in every time slot in every frame, wherein during such an empty time slot the channel is under utilized, because any other device waiting for transmission cannot transmit either during such a time slot in the present system.
As was presented earlier in this description, the radio local area network can comprise several access points, the scope of whose service area is influenced by the transmission capacity, ambient conditions, possible obstructions in the path of the signal, the directional pattern of the antenna, etc. In practice, the boundaries of the service area of the access points cannot be clearly defined, but the service areas of the access points located in the vicinity of each other overlap one another at least partly. Thus, in some cases, the wireless terminal can be located within the service area of two or more access points, but these access points do not necessarily detect the transmissions of each other and cannot synchronize with each other. The wireless terminal selects one of these access points to be used in the data transmission connection. On the other hand, the access point used in the connection at a given time can be changed when the wireless terminal is moved or when the quality of the connection varies, which is known as such. Because the access points can select the channel to be used in the connection irrespective of the other access points, and schedule the transmissions independently, it is possible that the properties of the data transmission between a wireless terminal and an access point used in the connection at a given time are influenced by one or more access points within whose service area the wireless terminal is located. Such interference can also occur in situations in which the frequencies to be used are not the same, but two access points transmit for example on adjacent channels.
The access points that interfere with the data transmission can also be access points of another radio network. This is possible especially in cases when in the same office building there are several radio network systems utilizing frequency ranges which overlap each other at least partly.
In addition to interference caused by other radio devices, the properties of the data transmission are also affected by changes in ambient conditions. These changes may be caused e.g. by the multipath propagation of the signal, by the movement of the wireless terminal within the service area of the communication network, from the area of one cell to the area of another cell, or outside the service area of the communication network, wherein the propagation conditions of the signal may vary. Also changes in the temperature and humidity of air may affect the propagation of the signal and cause changes in the data transmission connection. Such changes may cause changes in the strength of the received signal, i.e. fading. Thus, part of the signal may be attenuated so much that the receiver cannot find out the information transmitted in the signal, wherein the data transmission fails. Such attenuation can, however, be transient, and after a moment the connection may be restored to a sufficiently reliable level even without changing the access point and/or frequency level of the connection. Nevertheless, in radio local area networks of prior art, attempts are made to correct a communication failure caused by attenuation by finding out whether it is possible to select another channel or another access point for the data transmission connection. However, it is possible that the fading will also affect communication via another access point or on another channel.
One criterion for setting up a connection between a wireless terminal and an access point is the quality of service (QoS) desired for the connection. In some connections, for example in data connections, the rate of the data transmission is not as significant a criterion as the reliability of the data transmission. Thus, the data transmission parameters are selected in such a way that as reliable a data transmission as possible is achieved. On the other hand, for example when transmitting an audio and video signal, it is the real-time quality of the data transmission that is important, not the accuracy. For such a connection requiring realtime data transmission, it is possible to allocate several time slots in a frame, wherein a higher data transmission rate can be achieved. If necessary, it is also possible to allocate several channels for one data transmission connection.
When the data transmission is being set up, the wireless terminal is listening to find out which access points have signals to be received. The wireless terminal advantageously measures the strength of the signals and selects the access point whose signal is the strongest at the moment. Thereafter the wireless terminal and the access point conduct connection set-up signalling for instance to transmit parameters such as the required data transmission rate, connection type, data transmission channel, time slots and connection identifier which are used in the connection.
Typically also during the connection, the wireless terminal measures the strength of the signal of the access point used in the connection as well as the strength of the signals of the other possible access points within the coverage area. If it is detected that the signal strength of another access point is sufficiently greater than the signal strength of the access point used at that particular moment, it is possible to conduct a handover to this access point, which is known as such.
The purpose of the present invention is to attain more effective utilization of the radio resources than in radio network systems of prior art. Another purpose of the invention is to produce a more disturbance-free data transmission system when compared with data transmission systems of prior art. The method according to the present invention is characterized in what will be presented in the characterizing part of the appended claim 1. The wireless data transmission system according to the present invention will be characterized in what will be presented in the characterizing part of the appended claim 15.
The invention is based on the idea that the cause of data transmission errors is examined and reported to that unit of the communication system which performs the selection of the channel to be used each time and/or other radio management algorithms, such as power control and link adjustment. This unit can thus either make the channel selection or continue the transmission on the same channel. In the HIPERLAN/2 system, the access point is the unit in which the channel selection is made. In some communication systems, the channel selection can be made by the wireless terminal.
Using the present invention, significant advantages are achieved when compared with methods and wireless communication systems of prior art. Using the method of the invention, the degree of utilization of each communication channel can be increased and the need to change the channel can be reduced. Moreover, in the communication system of the invention, the channel is not gratuitously changed particularly in such situations in which the data transmission errors are primarily due to the phenomenon of fading. In the communication system of the invention, it is possible to further reduce the rate of interference, because the access points do not increase their transmission output unnecessarily, wherein the degree of utilization of the communication system is significantly improved.